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RF Cafe began life in 1996 as "RF Tools" in an AOL screen name web space totaling 2 MB. Its primary purpose was to provide me with ready access to commonly needed formulas and reference material while performing my work as an RF system and circuit design engineer. The World Wide Web (Internet) was largely an unknown entity at the time and bandwidth was a scarce commodity. Dial-up modems blazed along at 14.4 kbps while typing up your telephone line, and a nice lady's voice announced "You've Got Mail" when a new message arrived...

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Capacitor Dielectric Properties

The question arises all the time concerning which capacitor type to use for a particular application. This table provides a guideline for starters, but is in no way exhaustive. (DA = dielectric absorption)

The unofficial temperature coefficient designations for the capacitors are as follows: The temperature coefficient is given as "P" for positive, "N" for negative, followed by a 3-digit value of the temperature coefficient in ppm/°C. For example "N220", is -200 ppm/°C, and "P100" is +100 ppm/°C. The one exception in this system is "NPO" where an "O" instead of "0" is used, but quite a number of people use "NP0." In any event "NPO" means stable with temperature.

Dielectric Type	DA	Advantages	Disadvantages	Uses
NPO Ceramic (COG)	<0.1%	Tight tolerance High Q factor, low K Small case size Inexpensive Good stability Wide range of values Low inductance	DA generally low, but may not be specified Limited to small values (10 nF)	Low loss, timing and tuning applications
Monolithic Ceramic (High K)	>0.2%	Low inductance Wide range of values	Poor stability Poor DA High voltage coefficient
X7R (BX) (Barium Titanate)		Inexpensive Low DA available Wide range of values Smaller case size	Damaged by temperature >+85° C Loose tolerances High inductance	Bypassing, coupling, and frequency discrimination circuits
Z5U & Y5V		Smallest case size Very large values	Damaged by voltages >25 WVDC Very loose tolerances	Bypassing and coupling
Polystyrene	0.001% to 0.02%	Inexpensive Low DA available Wide range of values Good stability	Damaged by temperature >+85° C Large case size High inductance	Timers and filters
Polypropylene	0.001% to 0.02%	Inexpensive Low DA available High dielectric strength Wide range of values Negative TC	Damaged by temperature >+105° C Large case size High inductance	Stable oscillators and filters, sample and hold circuits and pulse handling circuits
Teflon	0.003% to 0.02%	Low DA available Excellent stability Operational >+125° C Wide range of values	Relatively expensive Large size High inductance	Timing and pulse shaping circuits
MOS	0.01%	Good DA Small Operational above +125° C Low inductance	Limited availability Available only in small capacitance values
Polycarbonate	0.1%	Good long-term stability Low cost Wide temperature range	Large size DA limits to 8-bit applications High inductance	Timers, filters and applications in high ambient temperature
Polyester	0.3% to 0.5%	Moderate stability Low cost Wide temperature range Low inductance (stacked film) Self-healing	Large size DA limits to 8-bit applications High inductance	Bypassing and coupling
Mica	>0.003%	Low loss at HF Low inductance Very stable Available in 1% values or better	Quite large Low values (<10 nF) Expensive
Aluminum Electrolytic	High	Large values High currents High voltages Small size	High leakage Usually polarized Poor stability Poor accuracy Inductive
Tantalum Electrolytic	High	Small size Large values Medium inductance High melting point High dielectric strength Good ductility	Quite high leakage Usually polarized Expensive Poor stability Poor accuracy

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